8 th Science

1. Chemical Reactions 8th Science

2. Vocabulary • molecule – combination of 2 or more atoms bonded together • compounds – substance made of 2 or more elements chemically combined • synthesis – 2 or more substances combined to make a more complex substance • decomposition – breaking down of compounds into simpler products • replacement – when element (s) in a compound (s) trade places

3. Getting Started • Signs of a chemical reaction • Color change • Odor • Precipitate formation • Release of a gas • Energy change • Exothermic reaction – heat given off • Endothermic reaction – heat taken in

4. Reaction needs energy to begin Once energy available atoms begin forming new bonds Minimum amt. needed is activation energy If exothermic -additional energy not needed to keep going as energy is being given off If endothermic -need constant supply of energy to keep going as energy is being absorbed Activation Energy

5. Changing the Rate • Reactions don’t all happen at the same rate • Speed can depend on conditions and how easily particles can get together • Reaction can be controlled by concentration, temperature, surface area and using catalysts or inhibitors

6. Changing Rates cont. • Increase concentration • Makes more particles available to react • Increase temperature • Heat makes particles move faster or particles come in contact more often due to more movement

7. More on Rate Changing • Change activation rate by adding catalyst • Allows need for less energy to make reaction happen • Catalyst not considered part of reaction • In humans have enzymes to allow reaction to occur at a lower temperature • Break material into smaller pieces more area exposed for reaction to occur faster • Inhibitor used to slow down reaction • Work in many different ways

8. Types of Reactions • Reactions classified by what happens to reactants and products • May add together, break apart, or exchange parts • Major types called combination, decomposition, or replacement • Can tell type by the chemical equation

9. Reaction in which 2 or more substances combine to form a single product, reactants may be elements or compounds, but product is always a single compound Example – sulfur trioxide and water to form sulfuric acid • SO2 + H2O H2SO4 Combination Reaction

10. Decomposition Reaction • Reaction in which single substance breaks down into 2 or more simpler substances, always just a single reactant in a decomposition • Example – breakdown of calcium carbonate upon heating • CaCO3 + heat CaO + CO2

11. Single Replacement • Reactions in which an element within a compound is displaced by a separate element, always has 2 reactants, one of which is always an element Zn + 2HCl ZnCl2 + H2

12. Double - Replacement • Reaction in which positive ion from one ionic compound exchanges with positive ion of another ionic compound, always have 2 compounds for reactants, usually occurs in aqueous solution and results in either formation of a precipitate, production of a gas, or formation of a molecular compound such as water AgNO3 + NaCl AgCl + NaNO3

13. Combustion Reactions • CH4 + 2O2 CO2 + 2H2O + heat + light • Can have combustion reactions • Are ones in which an element or a compound reacts rapidly with O gas to liberate heat and light energy, compounds combining with O in these reactions are hydrocarbons, fuels like kerosene and gasoline, complete combustion yields CO2 and H2O, if not have enough O combustion will not be complete – CO and C may be released

14. Lab 5 Will It React? Question: How can you identify a chemical reaction? Hypothesis: Materials: well plate HCl magnesium silver nitrate baking soda sodium hydroxide copperII sulfate mossy zinc distilled water pipettes paper towels laminated template goggles aprons Safety: Goggles must be worn at all times. Do NOT eat or drink anything during this lab. Wash your hands well before leaving class. Procedure:1. Notice the laminated reaction template taped to your lab table. This is where you will complete all chemical reactions. 2. Using the prepared and labeled pipet, add 2- 4 drops of HCl in the boxes identified on the template. Return the pipet to the holder. Make observations in Data Table 1.

15. Lab 5 continued 3.Place another 2-4 drops of HCl on the X in each box indicated where HCl is a reactant. 4. Repeat with 2-4 drops of copperII sulfate (CuSO4) in the box indicated on the template. 5. For each row that has a second reactant, add that reactant to the second column. Write your observations in Data Table 1. 6. Beginning with the first reaction, add the two reactants to the column marked “product” and observe what happens. Do only one reaction at a time, so you do not miss seeing any reaction that takes place. Record your observations in Data Table 1. 7. Continue combining reactants as indicated in the product column and write your observations for each set of chemicals. 8. When you have completed all combinations wipe the laminated template with a dry paper towel and through the paper towel in the trash can. 9. After you have cleaned the template and straightened the table be sure to wash your hands.

16. Data Table 1

17. Lab 5 cont. Results: Conclusion: • Compare and contrast the reactions of Al, Mg, and Zn with HCl. • Which reaction formed a precipitate? • Which reactions resulted in gas production? • Which reaction resulted in a color change? • Which reaction had no observable change? • What could you do to try to find out if any of these reactions resulted in a temperature change? • What differences did you see between HCl and CuSO4?

18. Lab 6 An Exothermic Reaction ? • Question: How can you tell that a chemical reaction is exothermic? • Hypothesis: • Materials: test tube test tube holder stirring rod bleach goggles thermometer hydrogen peroxide eye-dropper shredded steel wool • Procedures: • 1. Goggles and aprons are required for the lab. • 2. Get a clump of steel wool about the size of a marble and shred it. • 3. Put the steel wool into the test tube. There should be about 2cm of steel wool in the bottom of the test tube.

19. Lab 6 An Exothermic Reaction ? Cont. • 4. While using the clamp to hold the test tube, use the stirring rod to push the steel wool to the bottom of the test tube. Do not pack it, but make sure the wool is at the bottom. • 5. Pour enough hydrogen peroxide into the test tube so the steel wool is barely covered. • 6. Place the thermometer in the test tube. Make sure the bulb of the thermometer is in the hydrogen peroxide. After allowing the thermometer to adjust to the temp. record the starting temperature in Data Table 1. • 7. Once you have recorded the starting temperature add several drops of the bleach. As the chemical reaction is taking place record the temperature and record in Data Table 1. • 8. Once the reaction has stopped record the final temperature in Data Table 1. • Clean up the lab table. Pour out your mixture in the bucket in the front of the room and then rinse out the test tube for the next class.

20. Data Table 1

21. Lab. 6 Continued Results: Conclusion: • Graph your results. Does the graph show this to be an exothermic or endothermic reaction? • What did you observe when you covered the steel wool with the hydrogen peroxide? • What did you observe when the drops of bleach was added to the steel wool and the hydrogen peroxide? • What happened after several drops of bleach was added to the steel wool? • Did the temp. change? How much did it change? Did it go up or down? • Based on the graph and your observations is this an exothermic or endothermic reaction? How do you know?

22. Vocabulary cont. • activation energy – minimum amt. of energy needed to start a chemical reaction • concentration – amt. of one material in a given amt. of another material • catalyst – material that increases the rate of a reaction by lowering the activation energy • enzyme – a biological catalyst • inhibitor – material used to decrease rate of a reaction